The fluorescence protein phosphatase (PP-2A) inhibition assay was found to detect okadaic acid (OA) in mussels down to 1 mg/ 100 g of mussel tissue. It was more sensitive than the mouse bioassay (detection limit, 20 mg /100 g) or ELISA using the SCETI DSP check kit (detection limit, 10 mg / 100 g). A drawback of the PP-2A assay method has been its lack of sensitivity towards the esters of OA and DTX-1. This was addressed by including a hydrolysis step in the pretreatment of extracts which allows these derivatives to be converted to either okadaic acid or DTX-1 prior to the DSP assay. The method has been applied to the analysis of DSP in 21 samples of naturally contaminated mussels and the results from the PP-2A inhibition assay compared to those of HPLC. A good correlation was obtained for OA determined by the two methods in both unhydrolysed and hydrolysed samples. The new procedure will substantially reduce the incidence of false negatives in the DSP assay.